Fig. 4.

Compensatory response of the Drosophila tracheal system in response to a missing tracheal terminal cell (TTC). (A) One tracheal terminal cell (TTC) occurs in each dorsal branch. There are two dorsal branches per segment (one on the left; the other one on the right). (B) TTCs are stochastically lost in Drosophila larvae; in the example depicted in the scheme, the TTC corresponding to the dorsal branch of the 4th segment is missing. The TTC from the contralateral hemisegment extends cytoplasmic projections that cross the dorsal midline, invading the region lacking its own dorsal branch. (C) If the TTC contralateral to the missing branch is mutant for Sima, it fails to execute the compensatory response (i.e. it does not invade the hypoxic contralateral hemisegment). Instead, a TTC from the adjacent segment (the 3rd segment in the example) extends projections that provide oxygen to the region lacking its own tracheal branch.